1. Field of the Invention
The present invention relates to a method and a system for charged particle beam exposure in which a plurality of charged particle beam exposure apparatus placed in line are operated in parallel.
2. Description of the Related Art
In the prior art, an independent stage has been used for each charged particle beam exposure apparatus.
A charged particle beam exposure system has been proposed in which in order to reduce the placement space, a plurality of charged particle beam exposure apparatus is placed in line, a wafer is placed on the stage provided for each of the apparatus and the plurality of wafers is simultaneously exposed based on the same exposure data.
However, since the stages move in the same direction, slight vibration caused thereby is amplified. When the exposure pattern becomes extremely fine with the increase in degree of integration of semiconductor integrated circuits, slight vibration of the stages greatly affects the exposure position error.
To prevent the vibration, a method has been proposed in which, as shown in FIG. 8(A), two charged particle beam exposure apparatus 1A and 1B are placed in parallel to scan stages 19A and 19B so that barycenter G thereof do not shift. According to this method, as shown in FIG. 8(B), exposure is performed so that the exposure pattern on wafer 16A and the exposure pattern on wafer 16B are symmetrical with respect to point Q.
However, since two charged particle beam exposure apparatus 1A and 1B are merely placed in parallel, the total placement space cannot be smaller than the sum of the placement spaces of apparatus 1A and 1B.
An exposure system is considered in which a plurality of charged particle beam exposure apparatus is disposed in a line to form a multiple column, a sample chamber disposed below the multiple column is common to the plurality of charged particle beam exposure apparatus, one sample scanning stage is provided in the sample chamber and for a plurality of samples mounted on the stage, and exposure is simultaneously performed based on the same exposure data. According to this exposure system, since the exposure apparatus may be disposed closely to each other, it may be possible to reduce the placement space. In addition, since the same exposure data is used, the construction of the overall system may be simplified.
However, since the length of the stage increases in the direction of the line of the multiple column, it becomes more serious that the exposure position accuracy is decreased by variation in strain of the stage due to variation in temperature of the stage or variation in force exerted on the stage.
(1) Degradation of Exposure Position Accuracy Due to Temperature Variation of the Stage
For example, when five exposure apparatus are placed in a line with a pitch of 400 mm, the length of the stage is 2 m. When the stage is formed of alumina having a small density in order to operate the stage at high speed and to improve the stop accuracy, the linear expansion coefficient thereof is 4.times.10.sup.-6 /degrees. When the temperature of the stage changes by 0.01.degree. C., the elongation of the stage becomes EQU 4.times.10.sup.-6 .times.2.times.10.sup.6 .times.0.01=0.08 .mu.m.
For example, to form a pattern with a width of 0.5 .mu.m, the expansion due to the temperature change of 0.01.degree. C. cannot be ignored because an exposure position accuracy 1/10 of the width is required.
The temperature of the stage varies due to the irradiation of an electron beam onto the sample. It also varies due to friction at a guide for limiting the stage movement direction to guide the stage. The amount of the variation increases as the number of exposure apparatus placed in line increases. Further, the temperature of the stage changes due to variation in ambient temperature. It is difficult to restrain the temperature variation of the stage due to these causes to be 0.01.degree. C. or lower, so that the exposure position accuracy decreases. This problem becomes more serious as the exposure pattern becomes finer.
In the prior art, since the stage for a single charged particle beam exposure apparatus has been short, it has not been a big problem that the exposure position accuracy decreases due to the temperature variation of the stage.
(2) Degradation of Exposure Position Accuracy Due to Variation in Force Exerted on the Stage
The rigidity of the stage decreases as the length of the stage increases. In the charged particle beam exposure apparatus which generally employs the stage-in-lens type, the thickness of the stage for ensuring the rigidity of the stage is limited. On the other hand, as the length of the stage increases, the strain due to the tare weight increases, the linearity of machining decreases. Moreover, since the inertial force at the time of acceleration increases, the force received from the guide mechanism when the stage is driven increases to increase variation in strain due to variation in force exerted on the stage. Thus, as the length of the stage increases, variation in strain due to variation in force exerted on the stage increases.
When the stage is slightly rotated, Abbe errors are caused. For example, as shown in FIG. 9, assume that wafer 16 and reflecting mirror 70 are secured to a rigid stage (not illustrated). Laser beam LB is irradiated from a not-illustrated laser interferometric coordinate measuring equipment to point Q on the reflecting mirror 70 to measure the position of reflecting mirror 70. Charged particle beam EB is irradiated to point R0 which is an irradiating target position on wafer 16. A line passing through point R0 and perpendicular to wafer 16 and a line passing through point Q and perpendicular to reflecting mirror 70 intersect at point S0 at right angles. When the stage is slightly rotated about point Q by .theta. as shown in the figure, the points after the rotation corresponding to points R0 and S0 before the rotation are designated as R1 and S2, respectively, and the intersection point of the straight line of charged particle beam EB and wafer 16 after the rotation is designated as T.
Since point Q is the same before and after the rotation, the point of irradiation of charged particle beam EB onto wafer 16 changes from point R0 to point T due to the rotation even if the measurement values of the laser interferometric coordinate measuring equipment are the same. Since point R1 is the target position after the rotation, deviation .DELTA.1 of point T from point R0 is the exposure position error (Abbe error+.DELTA.0), where .DELTA.0 is the distance between point S1 and straight line R0.sub.-- S0 and is the exposure position error (Abbe error is 0) when h=0.
For example, when L=200 mm and h=10 mm, assuming that the deviation of point S1 with respect to point S0 is 5 .mu.m, EQU .theta.=5/(200.times.10.sup.3) EQU .DELTA.0=5.theta.=0.125.times.10.sup.-3 .mu.m EQU .DELTA.1-.DELTA.0=h.theta.=0.25 .mu.m.
Although .DELTA.0 may be ignored, error .DELTA.1 cannot be ignored. Consequently, the exposure position accuracy decreases due to the variation in force exerted on the stage. This problem becomes more serious as the exposure pattern becomes finer.
In the prior art, with assuming that a stage is overall rigid, rotational angle .theta. of the slight rotation of the stage is measured to correct the Abbe error. However, since a long stage cannot be regarded as being overall rigid, the correction accuracy is insufficient. As the exposure speed increases and the exposure pattern becomes finer, to make the correction, for example, at 10 MHz, it is necessary that the correction calculation can be performed in a short period of time and that a sufficient correction accuracy be ensured.